The present invention relates to a linear polarization/conversion apparatus suitably applied to a projection type color liquid crystal display for adding/mixing display images obtained by monochrome liquid crystal panels respectively arranged for red (R), green (G), and blue (B), and projecting a resultant color image.
With a growing trend toward larger television screens, a great deal of attention has recently been paid to a projection type liquid crystal display, which is designed to enlarge/project an image, displayed on a liquid crystal TV panel, on its screen, for it is small, light, and easy to handle. However, in comparison with image projection type CRT (cathode-ray tube) displays, which constitute the mainstream of projection type displays and have achieved a high degree of perfection, projection type liquid crystal displays are still unsatisfactory in terms of both resolution and brightness. That is, there is much room for improvement in projection type liquid crystal displays.
In order to increase the resolution, a high-definition liquid crystal TV panel is being developed. With regard to the brightness, it is assumed that a projection type liquid crystal display is superior to a projection type CRT display because it allows separation of an image formation portion from a light source. However, the brightness of a projection type liquid crystal display is still about 1/2 to 1/3 that of a standard CRT display. The quickest way to increase the intensity of projection light is to use a high-output lamp. This method, however, is not practical because degradation of the components of a display is caused by an increase in power consumption and apparatus temperature.
FIG. 4 shows a conventional projection type color liquid crystal display called a mirror scheme display using dichroic mirrors for color separation and mixing of light. Referring to FIG. 4, reference numeral 1 denotes a light source such as a xenon lamp. Light emitted from the light source 1 is reflected by a reflecting mirror 2 having a parabolic reflection surface and designed to convert source light into light parallel to the optical axis. The light is then converged toward a projection optical system 19 by condenser lenses (not shown) respectively arranged in front of liquid crystal panels 7, 12, and 15. At this time, the convergent parallel light is incident on a blue dichroic mirror 4 for splitting/reflecting only a blue light component. A blue light component 5 split by the blue dichroic mirror 4 is reflected by a mirror 6 in a direction parallel to the optical axis of the light source 1 and is incident on the transmission type liquid crystal panel 7. A voltage is selectively applied to the liquid crystal panel 7 in accordance with the constituent pixels of an arbitrary image to be projected. The blue light component 5 transmitted through the liquid crystal panel 7 is converted into a blue image light component 5a having an image signal.
The light, transmitted through the blue dichroic mirror 4, from which the blue light component 5 is lost, becomes a yellow light component 8. The yellow light component 8 is incident on a red dichroic mirror 9. As a result, a red light component 10 is split from the yellow light component 8. A remaining green light component 11 is transmitted through the mirror 9. The split red light component 10 is incident on the transmission type liquid crystal panel 12 having the same arrangement as that of the liquid crystal panel 7 so as to be converted into a red image light component 10a. The blue and red image light components 5a and 10a are mixed together by a mixing dichroic mirror 13 to generate a magenta image light component 14.
Meanwhile, the green light component 11 is incident on the transmission type liquid crystal panel 15 having the same arrangement as that of the liquid crystal panel 7 to be converted into a green image light component 11a. The green image light component 11a is then reflected by a mirror 16 to be incident on a mixing dichromic mirror 17. The green image light component 11a and the magenta image light component 14 are mixed together by the mixing dichromic mirror 17 to generate RGB added/mixed image light 18. The image light 18 is then enlarged/projected on a large screen 20 through a projection optical system 19, thus reproducing a color image.
FIG. 5 shows the actual arrangement (omitted in FIG. 4) of the transmission type liquid crystal panel 7 (identical to the liquid crystal panels 12 and 15). The liquid crystal panel 7 has two polarizing plates 21A and 21B arranged on both sides thereof. Such an arrangement is employed because the liquid crystal (twisted nematic liquid crystal) used for the liquid crystal panel 7 is designed to rotate the plane of polarization of incident light rather than to transmit or block light in accordance with the applied state of a voltage. More specifically, if natural light, which is polarized in random directions, is incident on a liquid crystal panel, natural light emerges from the panel regardless of the state of an applied voltage. Even if, therefore, an image is formed on the liquid crystal panel, the image cannot be recognized. For this reason, the polarizing plate 21A is disposed in front of the liquid crystal panel 7 to transmit only light components, of the natural light, which are polarized in a predetermined direction, thereby converting the natural light into linearly polarized light components. That is, when the natural light is transmitted through the polarizing plate 21A, the light is split into two linearly polarized light components orthogonal to each other. Of these light components, the light component parallel to the direction of polarization is transmitted through the polarizing plate 21A, while the light component perpendicular to the direction of polarization is absorbed. When the linearly polarized light component parallel to the direction of polarization is incident on the liquid crystal panel 7, the direction of polarization is partially rotated in accordance with an image, and the resultant light emerges from the liquid crystal panel 7. When only the light component polarized in a predetermined direction is transmitted through the polarizing plate 21B, a gradation image is obtained for the first time. Note that when the light component perpendicular to the direction of polarization is absorbed by the polarizing plate 21A, it is converted into heat.
As is apparent from the above description, in the conventional apparatus, since at least half of natural light is absorbed by the polarizing plate 21A in the process of extracting the linearly polarized light components from the natural light through the polarizing plate 21A, a problem is posed in terms of effective use of light. In addition, since the absorbed light is converted into heat to raise the temperature of the polarizing plate 21A, degradation of the polarizing plate 21A poses an additional problem. Under the circumstances, demands have arisen for the development of an apparatus which can convert all the light emitted from a light source into light components polarized in a predetermined direction, rather than extract a light component polarized in a predetermined direction through the polarizing plate 21A, thereby solving the above problems, i.e., an increase in light amount and degradation of a polarizing plate due to heat.
Note that the problem of degradation of the second polarizing plate 21B is not so serious as that of the first polarizing plate 21A, because the opening portion of the liquid crystal panel 7 is about 40% of all the area (an increase in opening degree is an important factor in terms of an increase in the brightness of a projection type color liquid crystal display).